Nearfault

Near-fault, often written nearfault in literature, refers to the region surrounding an active fault where earthquake ground motions exhibit distinctive characteristics during rupture. The term is used in seismology and earthquake engineering to distinguish near-source shaking from far-field shaking, with distances typically measured in tens of kilometers from the fault trace, though no strict boundary exists.

The distinctive behavior, known as near-fault effects, arises primarily from rupture directivity—motion caused by rupture propagating toward the site—along with fault geometry, slip distribution, and local site and basin conditions. These factors can produce unusually large, long-period velocity pulses and sustained shaking.

Common features of near-fault ground motions include pulse-like waveforms, especially in the velocity history, a pronounced long-period component, directional dependence, and longer duration. In some cases, permanent ground displacement or fling-step offsets can be observed.

Implications: Near-fault effects have important implications for building design and hazard assessment. They can produce higher demands on tall or flexible structures and influence probabilistic seismic hazard analyses. Engineers incorporate near-fault ground-motion models and site effects to improve design codes and retrofitting strategies.

Notable observations: The 1995 Kobe earthquake provided well-documented near-fault pulses; similar effects have been reported in other large events worldwide, leading to ongoing research into predictive models, scaling laws, and mitigation strategies.